The present invention relates to a biaxially oriented dielectric film of high-purity electrical film raw material containing propylene polymers and stabilizers for the propylene polymers. The invention further relates to the use of such a film.
The use of plastic films as electrical insulating material has been known for a long time. The films concerned are used both as a dielectric in capacitors and also for cable insulations or as self-adhesive insulating tapes. The requirements imposed on the electrical insulation properties of such films are very high. In addition to a low dielectric loss factor and a high breakdown strength, these characteristic data are required to be precisely constant with time, especially under operating conditions at elevated temperatures.
A series of publications is concerned with biaxially oriented polypropylene films which are said to have a particularly high breakdown strength. The good breakdown strength is generally achieved by using polypropylene raw materials which are said to have quite specific properties.
Thus, EP-A-0,011,796 describes a biaxially oriented polypropylene film for use as electrical insulating film which is composed of two layers and is used in metallized form for producing capacitors. The raw material for said electrical insulating film is high-purity polypropylene which has a residual ash constituent of less than 100 ppm, which is free of organic or inorganic lubricants and which does not contain any ionogenic constituents.
EP-A-0,222,296 has disclosed a coextruded, biaxially oriented multilayer film in which the base layer is likewise composed of high-purity electrical film raw material based on propylene polymers. To improve its electrical properties when used as a dielectric in capacitors, the stabilizer content of the outer layer(s) should be appreciably higher than the stabilizer content of the base layer. In connection with plastic electrical insulating films, stabilizers are to be understood to mean specific compounds which reduce the oxidation phenomena and consequently the dependence of the dielectric loss factor and the electrical breakdown strength on time, if said films are used as a dielectric in conjunction with thin metal layers as electrodes in capacitors. Despite the complicated multilayer structure, the known films still leave something to be desired in relation to the constancy of their dielectric properties, in particular for long-term loading under operating conditions at elevated temperatures.